A toner replenishment control structure operates optimally for developer materials which exhibit toning contrast characteristics which vary predictably with the concentration of toner particles in the developer mix, and for developer materials which do not exhibit predictable changes in toning contrast...http://www.google.com.au/patents/US4908666?utm_source=gb-gplus-sharePatent US4908666 - Apparatus for controlling toner replenishment in electrostatographic printer
